Dew point recorder and indicator



m.6,1942.; I c. w. THORNTHWAITER 2,268,785

DEW POINT RECORDER AND INDICATOR Filed June 13, 1941 8 El-// 5. 3 l "/7 29 2/ 26 /5 l6 4 3/ 1 7 1 /2 8 [3 I 7 Q Recorder I I I Inventor Char/es Warren Thomlhwa/fe Afforn eys Patented Jan. 6, 1942 Charles Warren Thornt hwaite, Arlington, 'Va.:

dedicated to the free use of the-People in the territory of the United States Application June 13, 1941, Serial No.39l,864 1 Claim. (01.73-51) (Granted under the act of March ,3, amended April 30, 1928; 370 ,0. G.

This application is made under the act of March 3, 1883, as amended by the act of April 30, 1928, and the invention herein described, if patented, maybe manufactured and used'by or for the Government of the United States of America for governmentalpurposes without the payment to me of any royalty thereon.

I hereby dedicate the invention herein described to the free use of the people in the territory of the United States to take effect on the granting of a patent to me.

' This invention relates to an apparatus for determining the moisture concentration of a gas by the method of ascertaining its dew point temperature. v

One of the objects of this invention is the provision of an apparatus which can be used to indicate the changes in themoisture concentration of a gas over periods of time.

This invention has many uses, such as in the held of weather forecasting. It can be used to determine evaporation from free water surfaces and evaporation and transpiration from land surfaces, which is of great importance in hydrology, agricultural climatology, ecology, and other studies. This inventionalso has commercial application in processes employing distilling or fractionating devices, such as in the manufacture of gasoline and alcohol. It is adaptable to the control of atmospheric humidity in air-conditioning work and may be used to prevent loss of perishable goods, such as foodstuffs and tobacco, during transportation by ship or railway.

This invention is further useful in determining the moisture concentration of gases flowing 1883, as 757) such as water ice with salt, dry ice with or'without alcohol, ether, and the like, or any other suitable-cooling medium. The other end of the rod 8 extends within the container 9 and is provided with'a knob or plate In incontact with the cooling medium. In this manner the rod I acts to transfer heat from its outerend' to the source of cold within'the'container 9.

Near themirror 5 there rod Ban electrical heating element II. The outer end of'the stopper I, together with the mirror 5, is enclosed in a light-proof, air-tight housing through pipes, such as municipally supplied illuminating gas.

The following description, considered together with the accompanying drawing, will disclose this invention more fully-its arrangement and operation of parts and further objects and advantages thereof will be apparent.

In the accompanying drawing there is illustrated a schematic diagram of an embodiment of my invention in which the numeral 5 designates a mirror or light reflecting surface, preferably of a heat-conducting metal, in contact with one end of a heat-conducting rod 6. The rod 6 is mounted through a heat-insulating stopper 1, such as cork, which stopper is.removably disposed in the mouth 8 of an inverted container 9, preferably of a heat-insulating material or construction, such as a conventional thermos bottle. The container 9 is adapted to hold a cooling medium,

I2, substantially as shown. A source of light It is provided, light from which is directed on the mirror 5 through a tube It mounted through a wall 15 of the housing 12. is dry, thelight from I specularly out of the housing I! through a tube It mounted through another wall [1 of the said housing opposite the wall It. Within the housingv l2, at a point which is out of direct contact with light emanating from the source ll or of light reflected specularly from the mirror 5, there is disposed a photo-electric tube it, or some other suitable light-sensitive device of the nonself-generating type, which is connected in series with the grid circuit of an amplification tube ll, said amplification, tube comprising the usual iilament 20, plate 2|, grid ,and cathode 23. A battery 24 is the source of potential for the photoelectric tube so 'that when light is incident on the photo-electric tube a current will flow in the circuit comprising the photo-electric tube It, the battery 24, and a resistor 25 which is in the grid circuit of the amplification tube It. The plate circuit of said tube is connected in series with v the heating element II and a 3" battery it. The potential for the filament III of the amplification tube is supplied by an A battery II. A C" battery 28 is usedin the usual manner to increase the negative potential on the grid 22 and todecrease the plate current so that, when the grid circuit is not connected to the photo-electric tube I8, no current flows through the plate circuit.

A thermocouple 29 is attached to the mirror 8 and is electrically connected to a suitable temperature-recording mechanism 30 of any type desired.

In operation the cooling medium is placed in the container 9 in contact with the knob or plate III to which heat flows from the mirror 5. Light from the source It is reflected specularly from the mirror 5 and out through'tube It. The flow of heat from the mirror 8 continues until moisis provided about the When the mirror 5 the source It is reflected I 'condenses on .the mirror ture from the atmosphere within the housing l2 5. The condensate causes the light reaching the mirror 5 to scatter or diffuse within the housing l2. Consequently, the photo-electric tube l8 receives a greater amount of light under this condition than it does when the light from the source I 3 is reflected specularly out through the tube l6 This increase in the amount of light which reaches the tube l8 consequently increases the current in the circuit comprising the tube It, the battery 24 and resistor 25, causing the grid 22 to become more positive with respect to the cathode 23 and, hence, increasing the current flowing in the plate circuit of the amplification tube l9 This causes an amplified current, sumcient to operate the heating element H, to flow through the plate circuit. The heat from said heating element is dissipated along the rod 6, some of which flows to the mirror 5 and evaporates the condensate thereon. As this evaporation progresses, the amount of light from the source l3 reaching the photo-electric tube decreases correspondingly, which, in turn, causes a decrease in the amount of heat from the heating element II. This proceeds to substantially a point of equilibrium where the amount of heat from the heating element I I balances the cold so that the temperature of the mirror 5'is maintained substantially at the dew point of the gas, a partial condensation at least remaining on the mirrior thereafter. As the moisture concentration in the atmosphere surrounding the mirror 5 changes, the temperature of condensation will vary, causing the equilibrium point to shift and causing the temperature of the mirror 5 to vary accordingly. By means of the thermocouple 20 and the recorder mechanism 30, changes over a period of time of the moisture concentration in the atmosphere surrounding the mirror 5 may be recorded. e

In using this device, the gas, the conditions of which are to be determined, is circulated through the housing l2 by means of a fan 3|,

which draws the gas in through the tube l6 and discharges it through an aperture 32 in the wall I. Any other suitable location of the aperture 32 may be provided, the general arrangement preferred being such that the gas entering through the tube It comes in contact with the mirror 5 as quicky as possible.

Having thus described my invention, I claim:

A device for determining the changes in moisture conditions of a gas comprising a source of cold, a heat conductible rod having one end in contact with said source, a plate having a lightreflecting surface normally contacting the other end of the rod, means for measuring the temperature of said plate, a source of light, a lightsensitive element, said reflecting surface reflecting light from said source away from said lightsensitive element while dry, said light-sensitive 5 means being disposed so as to receive light reflected from said reflecting surface when moisture has precipitated thereon, an electrical heating element near said plate, an amplification tube controlled by said light-sensitive element and means controlled by said amplification tube for: actuating said heating element only when light is incident on said light-sensitive element.

. CHARLES WARREN THORN'IYHWAITE. 

